1. Field of the Invention
This invention relates to methionine manufacture.
2. Description of the Prior Art
U.S. Pat. No. 2,527,366--Livak et al, issued Oct. 24, 1950, describe the preparation of methionine by hydrolyzing 5-(beta-methylmercaptoethyl)-hydantoin with an aqueous solution of barium hydroxide under pressure, e.g., within a bomb or autoclave, at temperatures above 115.degree. C. and usually within the range of from 115.degree. to 210.degree. C. Patentees mention that these conditions of temperature and pressure are not satisfactory for alkaline hydrolyzing agents in general. For instance, ammonium hydroxide and lime when evaluated as hydantoin hydrolyzing agents under similar temperature and pressure conditions, either caused excessive by-product formation and produced the alpha-amino monocarboxylic acid in an unsatisfactorily low yield or formed a badly discolored amino acid which could not be satisfactorily decolorized. Also, barium hydroxide itself, when tested as a hydrolyzing agent at atmospheric pressure, was unsatisfactory, that is, upon boiling a mixture of the hydantoin and aqueous barium hydroxide at atmospheric pressure, hydrolysis occurred very slowly and the yield of alpha-amino monocarboxylic acid was low.
Pierson et al, Jour. Am. Chem. Soc., 70, 1450 (1948) describe a three-step synthesis based on catalyzed addition of methyl mercaptan to acrolein, followed by the Bucherer hydantoin reaction and then by hydrolysis. Methionine was prepared from 17.4 g (0.10 mole) of 5-(beta-methylmercaptoethyl)-hydantoin by refluxing for six hours with a solution of 8.8 g of sodium hydroxide in 75 ml of water contained in a stainless steel flask. An additional 4.4 g of sodium hydroxide was added and refluxing was continued for a total of 24 hours. The reaction mixture was decolorized, neutralized to litmus with concentrated hydrochloric acid and cooled to 5.degree. C. to crystallize. A crude yield of 84.5% was obtained.
U.S. Pat. No. 2,557,913--Livak et al, issued June 19, 1951, disclose that 5-(beta-methylmercaptoethyl)-hydantoin may be hydrolyzed by heating the same together with an aqueous solution of an alkali such as sodium carbonate, potassium carbonate, sodium hydroxide or potassium hydroxide. However, they indicate that hydrolysis with most alkalies results in a low yield or in discoloration. They prefer barium hydroxide and use sufficient water to dissolve the major portion of the barium hydroxide and heat the hydrolysis mixture in a bomb or autoclave at a temperature of from 115.degree. to 210.degree. C. Air is preferably swept from the reactor with nitrogen, steam or other inert gas prior to heating the mixture under pressure since oxygen, if present during the hydrolysis, may cause by-product formation.